A mythical beast is an imaginary creature. Some such as the dragon, the griffin or the unicorn have their origin in traditional myth and have at one time been believed to be real creatures. Others were based on real creatures, originating in garbled accounts of travellers' tales; such as the "Vegetable Lamb of Tartary[?]", which supposedly grew tethered to the earth (and was actually the cotton bush). Examples of the semi-mythical creatures can be found in medievalbestiaries.

Conversely, some creatures once believed to be mythological have been found in recent times, such as the giant squid. As such, the boundries between what is recognised in human culture as being a mythological and existant creature have been subject to the limitations of human perception.